A novel acidic phosphoric-based geopolymer binder for lead solidification/stabilization

Many contaminated sites are acidic and the existing binders are mainly alkaline materials. Alkaline binders are used to treat contaminated sites in acidic environments and the solidification/stabilization (S/S) effect is affected by acid corrosion. Therefore, good application prospects exist in developing a binder suitable for the treatment of acidic contaminated sites. In this paper, a new acidic phosphoric-based geopolymer (named APG binder) was synthesized with fly ash as a raw material and aluminum dihydrogen phosphate as the reactant, and the APG binder was used for Pb2+ S/S for the first time. The pH of the APG binder with Pb2+ ranged from 2.56 to 4.09 during 7-28 days of curing, and its compressive strength with Pb2+ exceeded 10 MPa at 28 days. Moreover, Pb2+ had a significant impact on the APG binder compressive strength, and when the Pb2+ content was 0.6%, the APG binder yielded a maximum compressive strength of 6.5, 9.1 and 14.28 MPa at 7, 14, and 28 days, respectively. Furthermore, the compressive strength correlated well with pH and electrical conductivity. The proposed APG binder had a better S/S effect on Pb2+ than that of cement and alkali-activated geopolymers in acidic environments. The stabilization mechanism of the APG binder for Pb2+ included chemical precipitation, physical adsorption and encapsulation.